1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD) and a method for producing the same.
2. Description of the Related Art
A display is performed in LCDs by varying the electro-optical properties of the liquid crystal with respect to the orientation of the liquid crystal molecules in accordance with a voltage to be applied to the liquid crystal layer. There are various operating modes for LCDs: a dynamic scattering (DS) mode, a twisted nematic (TN) mode, a guest host (GH) mode, and a polymer dispersed (PD) mode using nematic liquid crystal droplets, for example. In general, these operating modes are roughly classified into two kinds of groups. Modes included in one group utilize the optical rotation phenomena caused by the liquid crystal layer, and modes included in the other group utilize the light scattering phenomena caused by the liquid crystal layer.
Among the LCDs operating in these modes, LCDs which operate in the modes utilizing the scattering phenomena of the light transmitted through the liquid crystal layer do not require polarizing plates for displaying, while LCDs which operate in the modes utilizing the optical rotation properties require a pair of polarizing plates for displaying. For LCDs operating in the GH mode (i.e. LCDs utilizing the anisotropy of the absorption coefficients of the dyes added to the liquid crystal), polarizing plates are required depending on the kind of operating mode of the liquid crystal and the kind of dye to be added. Recently, some LCDs utilizing the light scattering phenomena have polarizing plates so as to improve the viewing angle characteristics thereof.
FIG. 9 shows a typical conventional LCD 800 operating in the TN mode. As shown in FIG. 9, liquid crystal 3 is sandwiched as a display medium between a pair of substrates 1 and 2 facing each other. The substrate 1 has a glass substrate 1a. Pixel electrodes 4, bus lines 5 such as gate bus lines and source bus lines, thin film transistors (TFTs; not shown) and the like are formed on the surface of the glass substrate la facing the liquid crystal 3 so that the voltage applied to the liquid crystal 3 can be controlled for each pixel. An alignment film 6 is further formed over the pixel electrodes 4, the bus lines 5, and the like in order to align the liquid crystal molecules of the liquid crystal 3. The substrate 2 has a glass substrate 2a. A black mask 7 and a color filter 8 are formed on the surface of the glass substrate 2a facing the liquid crystal 3, and a protection layer 9 and a transparent electrode 10 are superposed on the black mask 7 and the color filter 8. An alignment film 11 is further formed on the transparent electrode 10 in order to align the liquid crystal molecules of the liquid crystal 3.
The substrates 1 and 2 are provided with polarizing plates 12 and 13 on the respective surfaces of the respective glass substrates 1a and 2a not facing the liquid crystal 3. The two polarizing plates 12 and 13 are disposed so that the transmission axes thereof are orthogonal to each other. Accordingly, the polarizing plates 12 and 13 shut out the light passed through the liquid crystal layer 3a to which a voltage is applied, but transmit the light passed through the liquid crystal layer 3b to which no voltage is applied. Alternatively, LCDs may perform display using a pair of polarizing plates disposed so that the transmission axes thereof are parallel to each other. In such a case, the polarizing plates transmit the light passed through the liquid crystal layer to which a voltage is applied, but shut out the light passed through the liquid crystal layer to which no voltage is applied.
Next, referring to FIG. 10, an LCD 900 for electrically controlling a transparent condition and a scattering condition by using of a multi-refraction of the light will be described below. A display medium 3' for the LCD 900 is composed of nematic liquid crystal droplets 3c dispersed in a polymer binder 3d. The LCD 900 operates based on the following principle. When no voltage is applied, the liquid crystal molecules included in the nematic liquid crystal droplets are in random orientation directions, and strongly scatter the light (a scattering condition), so that the display medium has a white opaque (or translucent) appearance as indicated by the circle 3b'. When a voltage is applied, the orientation directions of the liquid crystal molecules included in the droplets are aligned in a direction of an electric field generated by the voltage. If the ordinary refractive index of the nematic liquid crystal droplets is matched with the refractive index of the polymer serving as a support medium, then the transparent condition as indicated by the circle 3a' can be realized under an application of a voltage. On removing applied voltage, the orientation of each liquid crystal molecule returns into a random direction, so as to be in the light scattering condition (see, for example, "Liquid Crystal Applications and Uses Vol. 1", edited by Birendra Bahadur, World Scientific, p. 362). This kind of LCD conducts a display by shutting and transmitting the light based on an application of voltage in the above-mentioned manner.
For a conventional polymer dispersed liquid crystal display device (PDLCD) described above, polarizing plates are not necessary. However, in order to improve the viewing angle characteristics, an LCD has recently been developed in which the substrates 1 and 2 are sandwiched by a pair of polarizing plates 12 and 13, so that the transmission axes of the polarizing plates 12 and 13 are orthogonal to each other (see, for example, Japanese Laid-Open Patent Publication No. 4-212928).
Hereinafter, a process for producing a conventional liquid crystal display device shown in FIGS. 9 and 10 will be briefly described below. First, on the surface of the glass substrate la facing the display medium 3 (or 3'), bus lines 5 for transmitting input signals; TFTs (not shown) for controlling the signals transmitted through the bus lines 5; and pixel electrodes 4 for applying a voltage to the display medium 3 (or 3') are formed. On the other hand, on the surface of the counter glass substrate 2a facing the display medium 3 (or 3'), a black mask 7 for preventing the leakage of the light; a color filter 8 for color display; a protection layer 9 for protecting the color filter 8; and a transparent electrode 10 for applying a voltage to the display medium 3 (or 3') are formed.
In the LCD 800 operating in the TN mode shown in FIG. 9, polyimide is printed on the substrates 1a and 2a, and then the printed polyimide is rubbed to form alignment films 6 and 11 for aligning the liquid crystal molecules. In this way, each of the substrate 1 and the counter substrate 2 is formed.
Next, the substrate 1 and the counter substrate 2 are attached to each other. In the peripheral portion of either one of the substrates 1 and 2, a photocurable resin or a thermosetting resin is applied, thereby forming a seal member 14. An injection hole 15, from which liquid crystal is to be filled later, is provided in the seal member 14 as shown in FIG. 11. Subsequently, the substrates 1 and 2 are disposed so as to be opposed to each other. Gap materials (not shown) are provided between the substrates 1 and 2 for obtaining a desired cell gap. The seal member 14 is cured by irradiating the light through the substrates 1 and 2 onto the seal member 14 or by heating the substrates 1 and 2. After that, a display medium such as liquid crystal is injected through the injection hole 15, and then the injection hole 15 is closed.
Into the LCD 900 shown in FIG. 10, a homogeneous solution of photopolymerizable prepolymer (monomer or oligomer) with a liquid crystal is injected. In the same manner as in the LCD 800, the injection hole 15 is closed, and then the light is irradiated through the substrates 1 and 2, thereby allowing the prepolymer to polymerize and form a polymer phase 3d and a liquid crystal phase 3c (polymerization induced phase separation). Then, polarizing plates 12 and 13 are placed on the outer surfaces of the assembled substrates 1 and 2 in a manner that the transmission axes thereof are orthogonal to each other.
As described above, LCD substrates are conventionally made of glass. However, in these days, a flexible film is also used instead of a glass. Since a flexible film is thinner, lighter and less breakable than glass, an LCD having the substrates made of such a film can be thinner, lighter and less breakable.
In general, the polarizing plates are additionally attached to the LCD at the final production step. However, Japanese Laid-Open Patent Publication No. 61-221728 proposed an LCD having a substrate having polarization properties which are obtained by integrating a polarizing plate with the substrate. In this device, a dichroic dye is added to a polyester resin, and then the polyester resin is uniaxially extended, thereby forming a polarizing polyester film. After that, a transparent pixel electrode and a color filter are formed on a surface of the polyester film so as to form a counter substrate for the LCD. As a result, a number of layers required for constructing a substrate of such an LCD is reduced. This makes an LCD device thinner in size and its fabrication simple. In addition, such a device may provide a brighter display.